Isoflavone concentration(daidzein, glycitein, and genistein) of soybean(Taekwangkong and Sowonkong) were analyzed for studying the relationship between seed size and isoflavone concentration and seed coat color and isoflavone concentration. The isoflavone concentration was the greatest in daidzein then followed by genistein, however glycitein was the smallist of all. The isoflavone concentration was increased with the increasing of the ripening degree and seed size.